Mycoplasmas associated with or isolated from tissues of AIDS and non- AIDS patients have been compared biologically and serologically to a collection of reference and recently isolated strains of human origin maintained in this laboratory. Strains of Mycoplasma fermentans and M. pirum from AIDS patients were serologically similar to other reference strains of these species, and for the most part, were similar in biological characteristics. However, some M. fermentans strains were strongly hemolytic to human erythrocytes and some M. pirum strains, associated with human tumor cell lines, exhibited more active attachment features than AIDS-associated strains. All M. prium strains isolated to date possess an organized terminal tip structure, but most strains examined here did not hemadsorb nor express cytopathogenic effects on tissue cells. Five M. genitalium strains were all active in hemadsorption and attachment to glass and plastic surfaces. The potent cytadherence and cytopathogenic activity of these strains for selected lymphocytic and fibroblastic cell lines correlates well with their attachment properties and the presence of the specific adhesion protein found at the tip structure of these organisms. Mycoplasma serological testing of various human sera from patients with AIDS, from HIV-positive (AIDS-negative) patients, and from control patients were carried out with four Mycoplasma species (including the three species mentioned above and M. pneumoniae). A significant increase in high serum titers (above 1:128) to M. genitalium was found in AIDS patients (19.2%), over similar high titers in HIV-positive (6.3%) or control groups (5.0%). The relevance of this finding is uncertain at present and further serologic studies are in progress with these mycoplasmas.